1. Field of the Invention
The present invention relates to a semiconductor device provided with a function to control a current supplied to a load with a transistor. In particular, the invention relates to a pixel formed of a current-drive light emitting element of which luminance is changed by a current and a display device including a signal line driver circuit for driving a pixel.
2. Description of the Related Art
As a driving method of a display device using a self-light emitting element typified by an organic light emitting diode (also referred to as an OLED, an organic EL element, an electroluminescence (EL) element, or the like), a simple matrix driving method and an active matrix driving method are known. The former method has a simple structure, but has a problem such as difficulty in realizing a large and high luminance display device. In recent years, an active matrix method has been developed, in which a current flowing to a light emitting element is controlled by a thin film transistor (TFT) provided inside a pixel circuit.
It has been known that an active matrix display device has a problem that a current flowing to a light emitting element changes due to variations in current characteristics of driving TFTs, thereby causing luminance variation. That is, in the case of an active matrix display device, driving TFTs for applying a current flowing to a light emitting element is used for a pixel circuit, and it is a problem that characteristics of the driving TFTs vary, thereby a current flowing to the light emitting element changes and luminance varies. In view of this problem, various circuits for suppressing luminance variation without changing a current flowing to a light emitting element even when characteristics of driving TFTs in a pixel circuit vary are suggested (for example, see Patent Documents 1 to 4).
Patent Documents 1 to 3 each disclose a circuit configuration for preventing a change in current values flowing to a light emitting element due to variation in characteristics of driving TFTs arranged in a pixel circuit. The configuration is called a current-write pixel, a current-input pixel, or the like. Further, Patent Document 4 discloses a circuit configuration for suppressing a change in a signal current due to variation of TFTs in a source driver circuit.
FIG. 54 shows a first configuration example of a conventional active matrix display device which has been disclosed in Patent Document 1. A pixel in FIG. 54 includes a source signal line 5401, first to third gate signal lines 5402 to 5404, a current supplying line 5405, TFTs 5406 to 5409, a storage capacitor 5410, an EL element 5411, and a signal current inputting current source 5412.
Operations from writing of a signal current to light emission are described with reference to FIGS. 55A to 55E. A reference numeral denoting each portion in drawings correspond to those in FIG. 54. FIGS. 55A to 55C each schematically shows a current flow. FIG. 55D shows a relation of a current flowing through each path at the time of writing a signal current. FIG. 55E shows a voltage accumulated in the storage capacitor 5410 at the time of writing a signal current, that is, a gate-source voltage of the TFT 5408.
First, a pulse is inputted to the first gate signal line 5402 and the second gate signal line 5403 and the TFTs 5406 and 5407 are turned on. At this time, a current flowing through the source signal line, that is, a signal current is Idata.
Since the current Idata flows through the source signal line, the current path is divided into I1 and I2 in the pixel as shown in FIG. 55A. These relations are shown in FIG. 55D. Note that, it is needless to say that Idata=I1+I2 is satisfied.
A charge is not held in the capacitor 5410 yet at the moment that the TFT 5406 is turned on; therefore, the TFT 5408 is in an off state. Accordingly, I2=0 and Idata=I1 are satisfied. In other words, only a current for accumulating a charge in the storage capacitor 5410 flows in the meantime.
After that, as the charges are gradually accumulated in the storage capacitor 5410, a potential difference starts to be generated between both electrodes (FIG. 55E). When the potential difference between the both electrodes reaches Vth (point A in FIG. 55E), the TFT 5408 is turned on and I2 is generated. As described above, since Idata=I1+I2 is satisfied, a current still flows and a charge is further accumulated in the storage capacitor while I1 is decreased gradually.
The charge keeps being accumulated in the storage capacitor 5410 until the potential difference between the both electrodes, that is a gate-source voltage of the TFT 5408 reaches a desired voltage, that is a voltage (VGS) high enough to apply the current Idata to the TFT 5408. When the charge stops being accumulated (point B in FIG. 55E), the current I1 stops flowing and a current corresponding to VGS at that time flows to the TFT 5408; thus, Idata=I2 is satisfied (FIG. 55B), which leads to a steady state. Thus, a writing operation of a signal is terminated. At last, selections of the first gate signal line 5402 and the second gate signal line 5403 are terminated to turn off the TFTs 5406 and 5407.
Subsequently, a light emitting operation starts. A pulse is inputted to the third gate signal line 5404 to turn on the TFT 5409. Since the storage capacitor 5410 holds VGS which has been written before, the TFT 5408 is in an on state and the current Idata flows from the current supply line 5405, thereby the EL element 5411 emits light. Provided that the TFT 5408 is set to operate in a saturation region, Idata can keep flowing without change even when a source-drain voltage of the TFT 5408 changes.
In this manner, an operation to output a set current is hereinafter referred to as an output operation. As a merit of the current-write pixel, a desired current can be accurately supplied to an EL element because a gate-source voltage required to apply the current Idata is held in the storage capacitor 5410 even when the TFTs 5408 have a variation in characteristics and the like. Therefore, a luminance variation due to the variation in characteristics of the TFTs can be suppressed.
The aforementioned examples relate to a technique for correcting a change in a current due to a variation of driving TFTs in a pixel circuit; however, the same problem occurs in a source driver circuit as well. Patent Document 4 discloses a circuit configuration for preventing a change in a signal current due to a variation in the TFTs in the source driver circuit, which is generated in fabrication.
Further, a driver circuit of a light emitting element is known, which is provided with a current supply circuit (1) and a drive control circuit (2a) which have configurations where a current (Is) having the same current value as a current (Ir) flowing from a supply transistor (M5) which supplies a current to drive a light emitting element (EL) can be led to a drive control circuit (2a) through a reference transistor (M4), and a control can be conducted so that the current (Is) approaches a desired set current value (Idrv) and source-drain voltage data (Vs, Vr) become equal to each other based on the current (Is), the source-drain voltage data (Vs) of the reference transistor (M4) and source-drain voltage data (Vr, Vdrv) of the supply transistor (M5) (see Patent Document 5).
Further, a driver circuit is known, which is formed of a light emitting element provided in series between a first power source and a second power source; a driving transistor for driving the light emitting element; a first switching transistor for leading a control signal for controlling the driving transistor to a gate of the driving transistor; a differential amplifier for comparing a voltage at a connecting point of the light emitting element and the driving transistor, and a control voltage showing luminance of a pixel, which is inputted to the display device, in order to generate the control signal, and configured so that the control signal is led to the gate of the driving transistor through the first switching transistor (see Patent Document 6).
In this manner, in a conventional technique, a driver circuit is configured so that a signal current and a current for driving a TFT, or a signal current and a current which flows to a light emitting element when it emits light are equal or in proportion to each other.    [Patent Document 1] Japanese Translation of PCT International Application No. 2002-517806    [Patent Document 2] PCT International Publication No. 01/06484    [Patent Document 3] Japanese Translation of PCT International Application No. 2002-514320    [Patent Document 4] PCT International Publication No. 02/39420    [Patent Document 5] Japanese Translation of PCT International Application No. 2003-108069    [Patent Document 6] Japanese Translation of PCT International Application No. 2003-58106